The K-x has a very good noise profile for its class, including excellently balanced noise reduction in JPEG files. You can start to see a bit of degradation of detail in shadow areas at ISO 800 (look at the text under the bill), but even as high as ISO 1,600 I found the images quite acceptable. By ISO 3,200 the decrease in detail becomes obvious.
Pentax considers ISO 12,800 (and ISO 100) an expanded mode, though I'd probably add ISO 6400 to that as well. Though there's still usable detail at ISO 3,200, by ISO 6,400 noise and noise reduction artifacts are obvious even when scaled down. However, the K-x maintains good color consistency across all the ISO sensitivity levels.
You can see how nicely smooth and like traditional film grain the noise looks at ISO 1,600, and how you don't lose the quality of the highlights. (1/30 sec, f3.5, spot meter, ISO 1,600, AWB, Custom Image Natural, 18-55mm lens at 18mm)
I wouldn't recommend using ISO 12,800 as a rule, but this is actually pretty good, and not just for a sub-$800 model. You lose a lot of the shadow detail and there's obvious blotchiness, but the overall photo retains good saturation and it's more than acceptable for small Web use. (1/30 sec, f8, spot meter, ISO 12,800, AWB, Custom Image Natural, 18-55mm lens at 18mm)
Belle closed her eyes when I used flash, so I took a chance and bumped up the sensitivity; unfortunately, the LCD on the K-x isn't terribly reliable for making exposure judgments, so the original came out underexposed (left). I bumped up the exposure of the DNG file--Pentax supports that as well as its own PEF for raw--in Adobe Camera Raw, and though it resulted in the typical smeary, color noise artifacts on her tabby fur, the overall result was still better than usual. (1/100 sec, f4.5, spot meter, ISO 1,600, AWB, Custom Image Natural, 18-55mm lens at 42.5mm, exposure +1.3 in ACR 5.6)
Pentax's 18-55mm kit lens doesn't display a lot of distortion at its widest, and the body's internal distortion correction does a pretty good job of fixing what little there is. The dark spot in the upper left corner is a shadow, not vignetting.
(1/80 sec, f7.1, multisegment meter, ISO 100, AWB, Custom Image Natural, 18-55mm lens at 18mm)
The default sharpness setting is generally pretty good, though the kit lens itself could be a hair sharper. Because of the combination of poor shake reduction and a slow lens, I had to shoot this outdoors in sunlight instead of my typical indoor shot.(1/160 sec, f20, spot meter, ISO 200, AWB, Custom Image Natural, 18-55mm lens at 55mm)
Though the K-x's default image settings produce a slight color shift, despite ostensibly bumping up only contrast and sharpness, it's not nearly as severe a shift as in Sony's comparable models and unlike Sony Pentax provides a Natural option. However, Pentax's automatic white balance normally tends to be on the cool side, which results in an overall color cast.(1/160 sec, f5.6, multisegment meter, ISO 200, AWB, 50-200mm lens at 115mm)
It's unusual to see such a big difference among the metering results; usually, at least two of the options produce very similar exposures. That's probably how it should be, but I can't think of any reason the multisegment metering--the default--should have been quite so overexposed in this photo. (Shutter-priority at 1/160 sec, ISO 200, AWB, Custom Image Normal, 50-200mm lens at 160mm. f16 spot meter, f11 center-weighted, f8 multisegment.)